Atomic nuclei possess spin angular momentum which is dependent upon the nuclear spin quantum number, I. Many nuclei have I=O and possess no angular momentum. This includes all nuclei with both an even atomic and an even mass number. NMR principles can be applied to liquid and solid media containing atoms whose nuclei have non-zero angular momentum (spin) to perform in vivo spectroscopy. Therefore, when a nuclear magnetic substance, such as water, is placed in a homogeneous static field (with a magnitude H.sub.0), its resonance angular frequency .omega.0 is given by the equation: EQU .omega.0=.gamma.H.sub.0.
where .gamma. is the nuclear gyromagnetic ratio of a measuring substance and is a natural constant
Typically, in NMR spectroscopy, material being studied is subjected to both a static uniform magnetic field and a time varying radio frequency field. The result is the induction of nuclear magnetic resonance when the above-stated equation is satisfied. Thus, a particular nuclear magnetic resonance indicates the presence of selected nuclei in the sample.
The basic components of an NMR spectrometer are a magnet of very uniform field, a radio frequency (rf) source, for generating frequency through the range of all possible frequencies in the sample, and a detector of absorption of rf energy by the sample. Typically, the static magnetic field is produced by a suitable coil carrying a steady current, and in view of the magnitude of the magnetic field required, the coil may well be a superconducting coil, and the radio frequency field is produced by a supplementary coil or high frequency coil, supplied with high frequency current. Resonance is detected by a further or receiver coil surrounding the sample, or the supplementary coil can be time shared. The rf power from the transmitter is fed into the probe of the instrument. The probe is located in the magnet gap and houses the sample holder as well as the transmitter and receiver coils. The NMR signal is then detected by the receiver for further processes.
The atoms of a sample are excited when an rf pulse, with a frequency equal to the resonant frequency of those atoms, is transmitted to the sample coil. Pulsed NMR enables time resolved studies to be performed in vivo. During pulsed experiments, the NMR spectrometer is inactive most of the time, waiting for the spin system to relax before exciting it again.
In vivo NMR measurements from many different nuclei are now being used to study physiologic and medical problems (e.g., .sup.1 H, .sup.19 F, .sup.31 P, .sup.23 Na, .sup.7 Li, and .sup.13 C). Concurrent acquisition of in-vivo NMR data from more than one nucleus increases the information available from an experiment without requiring extra data collection time, while eliminating many of the problems of biological variability. The development of multinuclear probes capable of performing with high sensitivity is a prerequisite to collecting NMR data concurrently from any set of nuclei.
A major problem in implementing multinuclear NMR is the construction of a probe capable of operating at more than one NMR frequency. Several methods have been reported to double-tune NMR probes. These probes were primarily designed to provide a separate input to the coil at the second frequency for the purpose of decoupling. Modification of these circuits by tuning the inputs and connecting them to a single port provides a useful coil for two-nuclei NMR. A major problem with this approach is the large number of reactive elements needed to accomplish this which makes the probe very difficult and cumbersome to build.